1. Field of the Invention
The invention relates to a transistor structure. More particularly, the invention relates to a vertical transistor structure.
2. Description of Related Art
In order to accelerate the operating speed of integrated circuits and to meet customers' demands for miniaturizing electronic devices, physical dimensions of transistors in a semiconductor device tend to be reduced. With the reduction of the physical dimensions of transistors, the length of channel regions in the transistors is also reduced. Thereby, a severe short channel effect is likely to occur in the transistors, and an ON current of the transistors may decrease. A conventional solution to said issue is to enhance the dopant concentration in the channel regions. Nevertheless, this solution gives rise to an increase in the leakage current and therefore affects the reliability of devices.
Hence, to resolve said issue, a conventional horizontal transistor structure has been replaced by a vertical transistor structure in the industry. As such, the operating speed and integration of the integrated circuits are enhanced, and problems including the short channel effect are resolved. For instance, as disclosed by Fujimoto Kiyoshi et al. in Advanced Materials in 2007, a vertical organic thin film transistor (OTFT) can be formed due to the electrostatic repulsion between the polystrene particles that function as a blocking board. However, in the vertical OTFT, a depletion layer serving as an insulating layer is formed at the contact interface between aluminum and a semiconductor layer, and therefore voltages cannot be excessively applied. Besides, in the vertical OTFT, an ON-OFF current ratio is rather small, and an OFF current is overly high. Consequently, the conventional vertical transistor can still be significantly improved in terms of structural design and channel control, which is one of the main research topics in this industry.